The present invention relates to a passive optical network (PON) system in which a plurality of subscriber connection devices share an optical transmission line.
The PON connects an OLT (Optical Line Terminal) arranged as an optical access system at a station side and an ONU (Optical Network Unit) arranged as an optical access system at a subscriber side on one-to-n basis (n is an integer not smaller than 2) by using a device which passively performs optical signal multiplex/demultiplex such as an optical splitter. A plurality of ONU are respectively connected to subscriber terminals (such as PC) and convert electric signals from the terminals into optical signals and transmit them to the OLT. The optical splitter which have received the optical signals from the plurality of ONU optically (time division) multiplex the optical signals and transmit them to the OLT. Conversely, the optical signal from the OLT is branched into a plurality of optical signals by the optical splitter and transmitted to a plurality of ONU. Each of the ONU selectively receives and processes a signal destined to itself.
As has been described above, upstream optical signals transmitted from a plurality of ONU to the OLT is time-division multiplexed by the optical splitter. The OLT decides and reports the optical signal transmission timing to each of the ONU so that the optical signals from the plurality of ONU will not collide with one another and each of the ONU successively sends the optical signal at the timing received. Since each of the ONU is set at an arbitrary value in the range of optical fiber length, for example, 0 to 20 km, 20 km to 40 km or 40 km to 60 km as is defined in ITU-T Recommendation G. 984.1, Chapter 8 and Chapter 9, the distances between the OLT and the respective ONU, i.e., the optical fiber lengths may not be identical and the transmission delay times of the optical signals transmitted from the respective ONU to the OLT are also different. Accordingly, the OLT should decide the optical signal transmission timing considering the optical signal transmission delay time caused by difference in the distance to each of the ONU.
In order to realize this, the OLT uses the technique called ranging which is described in ITU-T Recommendation G. 984.3, Chapter 10. By using this technique, the OLT adjusts the transmission timing of the respective ONU as if they were at the identical or equal distance from the OLT, so that optical signals from the plurality of ONU will not interfere one another on the optical fiber. That is, the OLT decides and reports the optical signal transmission timing for each ONU by assuming that all the ONU are at an identical distance from the OLT. Furthermore, the OLT reports the optical signal delay time caused by the difference between the assumed distance and the actual distance where each ONU is located, to each ONU. Each ONU transmits an optical signal at the transmission timing reported from the OLT with the reported delay time.
Moreover, in order for a plurality of ONUs to share a communication band of a single optical fiber fairly and efficiently, the ITU-T Recommendation G. 983.4 defines the DBA (Dynamic Bandwidth Assignment) technique for the OLT to assign an ONU upstream band (data transmission position/time) in accordance with a request from each ONU. The OLT also performs bandwidth control based on this technique.